We present a study
of the lens properties of quadruply imaged systems,
lensed by numerically simulated galaxies. We investigate
a simulated elliptical and disc galaxy drawn from high
resolution simulations of galaxy formation in a concordance
ΛCDM universe. The simulations include the effects of gas
dynamics, star formation and feedback processes.
Flux-ratio anomalies observed in strong gravitational lensing
potentially
provide an indicator for the presence of
mass substructure in lens galaxies as predicted from CDM simulations.
We particularly concentrate on the
prediction that, for an ideal cusp caustic, the sum of the signed
magnifications of the three highly magnified images should vanish when the
source approaches the cusp. Strong violation of this cusp relation
indicates the presence of substructure, regardless of the global,
smooth mass model of the lens galaxy. We draw the following
conclusions: (1) the level of substructure present in simulations produces
violations of the cusp relation comparable to those observed; (2)
higher-order catastrophes (e.g. swallowtails)
can also cause changes of the order of 0.6 in the cusp relation
as predicted by a
smooth model; (3) the flux anomaly
distribution depends on the image parity and flux and both
the brightest minimum and saddle-point images are more affected by
substructure than the fainter images. In addition, the brightest
saddle point is demagnified w.r.t. the brightest
minimum. Our results are fully
numerical and properly include all mass scales, without making
semi-analytic assumptions. They are ultimately limited by the mass
resolution of single particles in the simulation determined by
current computational limits, however show that our results are not
affected by shot-noise due to the finite number of particles.

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